A Study of the CGM around an isolated galaxy in the Bootes Void
Mousumi Das, Indian Institute of Astrophysics, Bangalore ‘Bubbles Big and Small’ Meeting, IISc., June 11-14, 2018
Collaborators
Huib Intema (Leiden) Harsha Raichur K.S. Dwarkanath (RRI) Preeti Kharb (NCRA) Bhaibav and T.Ghosh (NISER)
Outline of talk
● Properties of void galaxies
● Cold gas and star formation in void galaxies
● The Bootes void galaxies
● Low frequency radio emission around AGN in the Bootes Void (at 610 and 150MHz) with the GMRT
● The extended X-ray emission around CG693
Voids
● Our universe is made of matter clustered along walls and filaments with large “empty” regions called voids in between. Voids and filaments clearly detected in the simulations.
● The voids have filaments is seen in both simulations as well as observations of the large scale structure.
(Haider et al. 2016)
Above : Simulations of cosmic web from the Illutrius simulation showing a cluster (on the left) and void region (on the right) (Haider et al. 2016.
Simulations on gas in voids
● Cosmological simulations such as the Illutris code have shown that a significant fraction of the baryon content at z=0 lies in the form of diffuse gas in voids (Haider et al. MNRAS, 2016).
● The gas could arise from AGN feedback at the void walls.
● Some of the gas can come from star formation and AGN activity from within the voids.
Haider et al. 2016 Void Galaxies
● They are gas rich, late type disk galaxies. Usually spirals and irregulars; ellipticals less common. Stellar masses of order 108 to 109 solar mass.
● Relatively blue and show signs of star formation. In the smaller voids the galaxies are usually low luminosity dwarfs or irregulars (Kreckel et al. 2011; Cruzen et al. 2002; Grogin and Geller 2001; Szomoru et al. 1997).
SDSS images of some bright galaxies in larger voids : SBS1428+529, VG_06, CG693-692
Groups/Interacting Pairs : Signatures of Void Substructure?
Kreckel et al. 2011 CG693-692 : Interacting pair in Bootes void There are many examples of interacting pairs, polar ring galaxies and even small groups of galaxies residing in voids. These galaxies may have formed when smaller voids evolved to form larger voids. This merging process can lead to the Triplet interacting system in a nearby void formation of filaments (Beygu et al. 2013) within larger voids – thus creating a void substructure.
Star Formation in Void Galaxies
In several surveys, void galaxies are found to be blue in color signifying star formation. Hα images and optical spectra also show signs of star formation in the gas rich spirals. On the color magnitude diagram for galaxy evolution , they fall mainly on the blue cloud. Thus void galaxies are not low luminosity systems as predicted but are slowly evolving galaxies.
Color magnitude diagram for galaxies Galaxy in the local void : NGC 6946. in the Void galaxy Survey (Kreckel et al. 2012).
Cold Gas in voids galaxies : CO(1-0) Detections
(Das et al. 2015)
SBS1325+597 (VGS_34) SDSS1538+3311 (VGS_57)
HCT Hα image (left) and SDSS g image (right)
HCT I band image
What triggers star formation in void galaxies?
NGC6946 : interacting at a 1. They could be interacting distance in our local void with close neighbors or with HI dominated galaxies that we do not see in optical images. 2. Gas accretion onto galaxy disks – the cold gas accretion makes the disks unstable and results in star formation. 3. The merging of sub-voids can result in galaxy interactions The position and increased gas accretion velocity plot of HI gas (e.g. Polar ring galaxy in void along the wall). galaxy minor axis. Off axis gas is at abnormal velocities.
(Boosma et al. ) Do void galaxies contribute to the warm/hot gas expected in voids?
● The hot gas gas can come from AGN outflows in the centers of clusters in the cosmic web.
● Galaxies within voids can also produce outflows associated with AGN activity and star formation
● Gas accretion onto galaxies from filaments in the IGM in voids We can explore diffuse emission in voids using low frequency radio observations. Also, X-ray emission will trace hot gas.
Why Bootes void?
● It appears to be the largest void – 60 Mpc across.
● Has the largest fraction of radio bright and star forming galaxies in nearby voids.
● It has probably evolved from merging of smaller voids – so greater chance of detecting extended gas associated with void galaxies.
Dey et al. 1990 Szomoru et al. 1990 Overview of the Bootes void galaxies
● Size is ~ 60 Mpc across (15 degrees), mean redshift ~0.05.
● About 28 galaxies reported in literature. Generally HI rich, late type spiral galaxies.
● AGN fraction (Seyferts, LINERS) high - about 20% compared to 3% for galaxies in other voids.
● A significant fraction are bright and star forming (Cruzen et al. 1997; Szomoru et al. 1996); several are in pairs or triplets. But overall region is very under dense.
Examples of Bootes void galaxies Determining Galaxy filaments in the Bootes void using method by Tempel et al. (2014)
● To determine whether the Bootes void has more galaxy filaments compared to other void galaxies, we made two sample : non-Bootes void galaxies 128 in number (Kreckel et al. 2012; Grogin & Geller 2000) and 28 Bootes void galaxies.
● We used the code based on the Bisous technique (Tempel et al. 2014) to determine galaxy filaments > 1Mpc in length.
● We found that 14/23 (11%) of non-Bootes void galaxies lie along filaments but 9/28 (39%) of Bootes void galaxies lie on filaments.
● This strongly suggests that larger voids such as the Bootes contain more galaxy filaments that arose from merger of subvoids.
Goals of low frequency radio study : to detect extended low frequency emission in and around void galaxies (235, 610MHz)
● To study emission associated with star formation/and or AGN activity around galaxies using low frequency (610M/ 235 MHz) radio continuum observations.
● Compare with hot gas associated with AGN activity or high mass star formation using X-ray emission.
● Identify regions to search for filaments in voids that can be done with more sensitive arrays such as the SKA.
● probe primordial magnetic fields.
Low frequency radio observations of galaxies in the Bootes void with the GMRT
● We have done 610 and 240 MHz observations of the radio emission around the 4 bright AGN host galaxies in the Bootes void.
● Observations were done in November, 2014 over 2 days. Total observing time was 14 hours for 4 galaxies : CG692- 693, SBS1428+5255, Mrk845, IZw81.
Sample of void galaxies in our radio study
CG693-692 MRK845 Z=0.056 Z=0.044
SBS1428+529 Z=0.044 IZw081 Z=0.052
610-235 MHz emission around Mrk845
Mrk845 is an emission line galaxy with Sy2 type AGN. It is an isolated spiral galaxy and has a regular disk structure. Flux at 610MHz is 3.4mJy . It has been detected in ROSAT and has an X-ray flux of LogLx=43.7. We are processing the archival Chandra data. It is detected in TGSS ADR. SDSS Color Composite image
610-235 MHz emisison around MRK845 and optical comparison
610 MHz 235 MHz At 610 MHz, radio emission is fairly compact. At 235 MHz the emission around MRK845 is extended well outside the optical radius. SDSS g band SDSS z band
610 MHz emisison around SBS1428+529
SBS1428+429 has a Sy2 type AGN. It is an isolated spiral galaxy and has a regular disk structure. Not detected in TGSS. Flux at 610Mhz is 5.1mJy. It has been detected in ROSAT and has weak X- ray flux.
SDSS Color Composite image
610-235 MHz emisison around SBS1428+529 and optical comparison
610 MHz 235 MHz At 610 MHz, radio emission is fairly compact. At 235 MHz the emission appears extended but it may be an effect of the larger beam. SDSS g band SDSS z band
610 MHz emisison around IZw81 SDSS I band image It is a small disk galaxy with a Sy2 type AGN. It is an isolated spiral galaxy and has a regular disk structure and a ring or spiral arm. It is the only AGN host galaxy that lies along a filament.
Flux at 610Mhz is 4.3 mJy. Not detected in TGSS 150 MHz map. No x-ray emission.
SDSS Color Composite image
610-235 MHz emisison around IZW081 and optical comparison
610 MHz 235 MHz At 610 MHz, radio emission is uniformly extended around the galaxy. At 235 MHz the emission appears similar. This may be due SDSS g band SDSS z band to AGN outflows.
610 MHz emisison around the galaxy pair CG692-693
● CG692-693 is a pair of closely interacting galaxies. CG693 has a Sy1 nucleus whereas CG692 is a star forming galaxy. Bright in X-ray (ROSAT).
● At 610 MHz we detect a total flux ~5.6mJy around CG692 and ~0.54mJy around CG693. At 1.4 GHz the flux (FIRST) is ~9mJy for CG692 and 2.2mJy for CG693. SDSS color composite ● It is just detected in TGSS image (Intema et al. 2016).
610 MHz and 235 MHz emisison around CG692-693
GMRT 610 MHz image (left) and 235 MHz image (right)
610-235 MHz emisison around CG692-693 and optical comparison
At 610 MHz, 610 MHz 235 MHz CG692 is prominent but the companion CG693 is weak. The emission around CG692 extends well outside the optical radius. CG692 appears to be an interacting SDSS g band SDSS z band system with two nuclei.
Spectral Index of the 610-235 MHz emission
● We calculated the spectral index of the 610-235 MHz emission using the peak flux, where S ~ ν-α ν
● CG692 α = 1.3
● MRK845 α = 0.70
● SBS1428+529 α = 0.60
● IZw081 α = 0.85
● The spectral indices are steep and consistent with outflows due to star formation and/or nuclear activity.
Preliminary Magnetic field estimation from radio luminosity of the void galaxies
● The radio emission is synchroton emission and associated with a magnetic field B.
● If we assume a minimum energy for the radiation then we can get an order of magnitude for the B
Galaxy name size of emitting L(610) B(min) region (kpc) (1022W/Hz) ( µG) CG 692 26.2 3.9 2.6 CG 693 8.4 1.5 5.3 Mrk845 13.1 1.6 3.7 SBS1428+529 12.9 2.4 4.2 IZw081 14.5 2.6 3.9
X-ray emisison around void galaxies?
Only a few void galaxies have been detected in ROSAT and very few have targeted X-ray observations (Anderson et al. 2007) . In the Bootes void 5/28 galaxies are detected in X-ray. But only one galaxy has significant emission – CG693.
Chandra X-ray emisison around CG692-693
Among the 28 Bootes void galaxies only 3 have been detected and imaged in X- ray. Of them, CG693 has the largest flux in X-ray. In the Chandra image the emission is concentrated on the Sy1 nucleus in CG693. Extends to twice optical size (~50kpc). Exposure time 18.8 ks. LogLx=42.9 The extended emission could be the circum-galactic medium (CGM) around the galaxy and is fed by the AGN. There is no X-ray emission associated with the star forming companion CG692.
The optical spectra of CG693 and CG692
The SDSS optical spectrum of CG693 shows broad emission components, typical of Seyferts and an outflow associated with the AGN. The SMBH mass is 107 M o The spectrum of CG692 has emission lines associated with star formation, and is classed as a starburst galaxy SFR(Hα)=28 M yr-1. o
CG693 CG692
Chandra X-ray emission around CG692-693
In the Chandra image the emission (0.5 to 7kev) shows some alignment towards CG692. Also some filaments in center.
The spectrum does not have any emission lines.
Comparison of 610, 235 MHz radio and Chandra X-ray emission around CG692-693 610 MHz 235 MHz
When the X-ray contours are overlaid on the 610 and 235 MHz images, the is a clear distinction. The radio emission is due to star formation but the X-ray is due to the Sy1 activity. Chandra X-ray emission around Mrk202
● The elliptical galaxy Mrk202 from the Void Galaxy Survey (kreckel et al. 2012) shows broad emission lines in optical spectrum, x-ray emission in Chandra observations and emission in its x-ray spectrum.
Chandra X-ray emssion overlaid on SDSS Chandra X-ray spectrum I band image
Our main results
● We have detected low frequency radio emission around AGN host galaxies in the Bootes void. If these AGN evolved during void mergers then they are good future candidates for detecting CGM/WHIM in voids using more sensitive arrays (e.g. SKA).
● Of the 4 galaxies that we detected at 610 and 235 MHz, the galaxy pair CG693-692 shows the largest radio and X-ray emission.
● CG693 has extended (~50kpc) X-ray emission which forms its CGM. It is fed by the Sy1 AGN. CG692 has a radio halo emitting synchroton emission at 1420, 610 and 235 MHz.
● It is possible that gas outflows from AGN within voids can contribute to some of the hot gas within voids, although finally it will depend on the void size.